This invention relates to the field of fiber reinforced materials. It is known in that field that filaments of material may be combined with a matrix to produce a composite material that exhibits desired qualities of the constituent materials and frequently some qualities not possessed by the individual constituents as such.
One method of making fiber reinforced composite material is to combine the fibers in layers or laminae in each of which the fibers are oriented in such a manner as to impart desired properties to the finished product. For example, a composite made with carbon filaments oriented in one direction exhibits higher stiffness properties in that direction but exhibits properties more typical of the matrix in the direction perpendicular to that in which the filaments are oriented. By adding one or more layers of fibers that are oriented perpendicular to the fibers in the original layers, the stiffness properties can be improved in the direction of orientation of the fibers comprising the added layers as well. Such a composite is then said to have orthogonal fiber orientation. One of the limitations of such orthogonally reinforced composites is that their resistance to shear deflection is not dominated by the properties of the fibers but rather by those of the matrix material. Typically, this is corrected by adding plies in which the fibers are oriented at an angle other than that of one of the orthogonal directions. The effect is analogous to that of a cross brace in a screened door or a wind brace in a building structure, as the shear or racking resistance then becomes a function of the properties of these off-angle reinforcements. The governing principles for predicting the properties of such multidirectionally reinforced composite materials are known per se.
While successfully addressing some of the difficulties as noted above, fiber reinforced composite materials of this type, which are generally referred to as a "laminated composite" materials, still exhibit several undesirable characteristics. The properties of such composites in directions which are more or less perpendicular to the plane of the base plies previously described are still matrix, rather than fiber, dependent and thus exhibit the comparatively less desirous characteristics of the matrix. Further, the fibrous layers cannot be made to intersect. Therefore, any structural shape having intersecting planes of laminated composite material will continue to be dominated in the region of intersections by the comparatively more limited characteristics of the matrix rather than of the fibers. An example of such a structure, useful to reinforce structurally a reinforced composite panel against deflection in directions normal to the plane of the panel, is one in which rib type stiffener members located at one surface of the panel are attached to it and to each other, effectively subdividing the panel into smaller panels. Since the fibrous layers of any given rib cannot be made to intersect with those of the panel or the other ribs, each interface between them effectively becomes matrix dependent for its properties because the matrix material itself, and not fibrous material, becomes the transition material between the overlying fibrous layers in the regions of such interfaces. Since the strength and stiffness properties of the fibers frequently are as much as 20 times higher than those of the matrix material, these limitations become serious obstacles to the effective utilization of the properties of the fibers.
An alternative to using laminated plies of reinforcing fibers is to utilize so-called Three-Dimensional ("3-D") weaving methods to fabricate a "preform" of integrally woven strands of fibers. In this context, the term "Three-Dimensional" means that, as contrasted with the usual weaving or other positioning of textile strands and/or fibers in substantially planar arrays, textile constituents are included in the form of strands and/or fiber arrays oriented at an angle with respect to such planar arrays. This may be achieved by weaving the fibers materials of the planar array in undulating fashion about vertical rods located where reinforcing ribs or other intersecting walls are to be positioned. The portions which are to become the desired walls or ribs are built up with the textile materials to produce the "3-D" or "Three-Dimensional" portion of the structure. Later, the rods are removed and replaced by yarns or other textile fiber arrays that are pulled or woven through the channel-like voids that had been occupied by the rods. A version of this type of structure is an orthogonal Three-Dimensional "weave", where each of the three directions is orthogonal to the other two directions. A textile structure so made usually is fabricated first, and the matrix material subsequently injected into it. The base textile structure before matrix infiltration is usually called a fiber "preform". Such orthogonal, Three-Dimensional preforms can overcome one of the limitations of a prior art laminated composite by enabling planes of material to intersect with each other with reinforcing fibers penetrating and becoming integral with both planes at each intersection or abutment. Although this can solve the matrix dominated intersection problem, it does not result in fiber orientations that effectuate high shear properties. It has previously been noted that the shear properties can be improved in any one plane by adding fibers that are oriented in one or more directions other than those of the other yarns (which are usually vertical and horizontal) within that plane. In the case of a rib stiffened panel, this can be used to cause the panel to have both good orthogonal properties and good shear characteristics; i.e., to have "quasi-isotropic" properties. However, the properties of the ribs themselves would still be dictated by and have the previously described limitations resulting from their substantially orthogonal fiber orientation in the plane perpendicular to the panel. Thus, this method of preform fabrication does not produce fiber orientations necessary to achieve quasi-isotropic properties in two intersecting planes.
To overcome these objections, the constituent laminae might be stitched together with enough stitching density to improve the properties perpendicular to the laminae. However, this approach also has drawbacks, including damage to the reinforcing fibers in the laminae, inability to incorporate high stitching densities near the intersections of the planes in the preform, and not resolving the discontinuities in the laminae where they join.
Another approach to overcoming these objections might be to combine "3-D" weaving techniques with lamination methods by attaching plies of woven fabric to the exterior of the 3-D preform, with the objective of having the resulting combination exhibit desired properties simultaneously in both the laminated and 3-D portions of the composite structure. This might be done by so orienting the fibers constituting the woven fabric plies that they compensate for the orthogonal limitations of the 3-D perform. A principle limitation of this approach is in attaching the plies to the outside of the preform rather than distributing them through the thickness. This is not good design practice, and it becomes more objectionable as the thickness increases.
Accordingly, it is an object of this invention to produce preforms for fiber reinforced composite structures.
Still another object of this invention is to produce such preforms utilizing so-called "Three-Dimensional" weaving techniques.
Yet another object of this invention is to produce such preforms which will satisfy one or more of the foregoing objectives in which non-orthogonal reinforcing fibers are interspersed with the orthogonally oriented fibers.
Still another object of this invention is to produce such preforms which will satisfy one or more of the foregoing objectives and include reinforcing fibers oriented so as to resist shear deflection in each of multiple intersecting and abutting planes.
Another object of this invention is to produce such preforms which will satisfy one or more of the foregoing objectives and include fibers continuous through the intersections of intersecting and abutting planes of the preform to allow tension and compression stresses to be transmitted through such intersections by fibers in addition to matrices.
Still another object of this invention is to produce such preforms which will satisfy one or more of the foregoing objectives and include ribs or other stiffeners attached a panel and/or other elements of the preform by fibers which are integral with both.
Yet another object of this invention is to produce fiber reinforced composite structures in which preforms which satisfy one or more of the foregoing objectives are incorporated into a matrix material.